The present invention relates in general to object geolocating and tracking systems of the type described in U.S. patent to Belcher et al, U.S. Pat. Nos. 5,920,287, and 5,995,046 (hereinafter referred to as the ""287 and ""046 patents, respectively), assigned to the assignee of the present application and the disclosures of which are incorporated herein, and is particularly directed to a reduced complexity, non-modulated AC magnetic field communication scheme. This non-modulated AC magnetic field communication scheme includes a (xe2x80x98demodulatorlessxe2x80x99) magnetic field detector, which is readily coupled with a tag transceiver unit, and responds to a narrow-band, non-modulated AC magnetic field used to communicate with the tag when the tag comes within a prescribed proximity of a tag-programming magnetic field generator.
The general architecture of the radio tagged object geolocation systems described in the above-referenced ""287 and ""046 patents is diagrammatically shown in FIG. 1 as comprising a plurality of tag emission readers 10 geographically distributed within and/or around an asset management environment 12. This environment contains a plurality of objects/assets 14, whose locations are to be monitored on a continuous basis and reported to an asset management database 20, which is accessible by way of a computer workstation or personal computer 26. Each of the tag emission readers 10 monitors the asset management environment for RF emissions from one or more RF-transmitter-containing tags 16 that are affixed to the objects 14. Each tag""s transmitter is configured to repeatedly transmit or xe2x80x98blinkxe2x80x99 a very short duration, wideband (spread spectrum) pulse of RF energy, that is encoded with the identification of its associated object and other information that may be stored in a tag memory.
These blinks or bursts of RF energy emitted by the tags are monitored by the readers 10, which are installed at fixed, and relatively unobtrusive locations within and/or around the perimeter of the environment being monitored, such as doorway jams, ceiling support structures, and the like. The output of each tag reader 10 is coupled to an associated reader processor. The reader processor correlates the spread spectrum RF signals received from a tag with a set of spread spectrum reference signal patterns, to determine which spread spectrum signals received by the reader is a first-to-arrive RF spread spectrum signal burst transmitted from the tag.
The first-to-arrive signals extracted by the reader output processors are forwarded to an object location processor within the processing subsystem 24. Using time-of-arrival differentiation of the detected first-to-arrive transmissions, the object location processor executes a prescribed multilateration algorithm to locate within a prescribed spatial resolution (e.g., on the order of ten feet) the tagged object of interest.
In their normal mode of use, the tags exhibit a prescribed operational functionality, such as transmitting or xe2x80x98blinkingxe2x80x99 at a relatively slow repetition rate. The use of a relatively slow blink rate is due to the fact that most of the objects being tracked do not move frequently. However, there may be occasions where it is desired to change the operation of or otherwise communicate information to a tag, such as stopping the tag from blinking or causing it to start blinking, or to transmit additional data, such as that acquired from optional sensors or a data bus.
As another illustration, there are times when the objects to which the tags are attached are moved and may pass through one or more regions of the monitored environment where communications with the tags are desired. For example, the monitored environment may contain xe2x80x98increased sensitivityxe2x80x99 regions (such as doorways and the like) where more frequent tag transmissions are desired, in order to ensure that any objects passing therethrough can be readily tracked. One way to accomplish this particular task would be to simply program the tags to blink more frequently on a continuous basis. However, this approach is not acceptable for two reasons. First, more frequent tag transmissions on a continuous basis will shorten the battery life of the tag; secondly it would increase spectrum congestion.
In accordance with the invention disclosed in the above-identified ""290 application, the above-described tag-reprogramming function is readily achieved by placing an arrangement of one or more relatively short range, modulated magnetic field proximity-based, tag-programming xe2x80x98pingersxe2x80x99 at a respective location of the monitored environment that is proximate to a region (such as a doorway) through which a tag may pass. This tag-programming pinger arrangement is operative to emit a non-propagating, modulated AC magnetic field, that is modulated with frequency shift keyed (FSK) encoding signals representative of digital data to be transmitted to the tag.
Such encoding signals may include, but are not limited to programming information, data or a stimulus, to be modulated AC magnetic field-coupled to any tag passing through that region. As a non-limiting example, tag reprogramming information may be used to cause the tag to immediately begin blinking at an increased rate for a relatively brief period of time, so as to alert the tracking system of the presence of the tag in the region. While the use of an AC magnetic field FSK-encoded using operational frequencies that are typically less than a few hundred KHz allows a large amount of data to be rapidly communicated to the tag, such frequencies are dominated by man-made and natural noise levels that limit performance.
In accordance with the present invention, the unwanted influence of such a noise-source environment upon an FSK-encoded magnetic field-based communication system is substantially diminished, by simplifying the complexity of the communication scheme to obviate the need for bandwidth that is unnecessary to achieve the transfer of a more limited amount of information. As opposed to an application in which the purpose of the communication link is to transmit xe2x80x98dataxe2x80x99 (where a fairly large (data) bandwidth may be required and increases susceptibility to interference), the present invention is intended for an application where the magnetic field communication link is used for a relatively simple function that can be represented by a single frequency. An example of such a simple function includes proximity detection or transmitting a relatively simple command, where the bandwidth requirement is considerably decreased, allowing a reduced complexity communication implementation.
Pursuant to the first embodiment of the invention, a magnetic field-sensing coil is coupled to a downconverter referenced to a highly stable local oscillator producing an AC magnetic field frequency (e.g., on the order of decimal fractional multiple of 100 KHz) that is readily distinguishable from other normal factory background electrical and electronic noise. The output of the downconverter is therefore a very low frequency. This (baseband) frequency is filtered in a very narrowband lowpass filter, and applied to a valid AC magnetic field tone detection path, and a range detection path. In the valid field tone detection path, the 100 Hz signal is xe2x80x98squaredxe2x80x99 by a hard limiter and applied to a digital tone filter, which counts transitions in the narrowband signal over a prescribed detection/integration interval to determines whether a valid AC magnetic field signal has been detected. The digital tone filter may alternatively be configured to operate against a plurality of tones, to accommodate reception of plural AC tone signals.
In the range detection path, which provides an analog representation of the received magnetic field signal level, the filtered signal peak-detected and coupled to a range-associated threshold detector, which indicates whether the tag is within a prescribed proximity of the AC source. This range detection output is logically combined with the tone detect signal to validate detection of the AC magnetic field tone and also that the tag is within a predetermined proximity of the AC tone field generator.
In a second embodiment of the magnetic field sensing unit, the detected signal output is digitized and processed by a digital processor which performs both digital tone detection and amplitude detection on the sample signal. The digital processor based embodiment has the ability to detect the AC field tone in the presence of noise that is hundreds of times greater than the AC field tone. This is achieved by operating components upstream of the digitizing section in a linear mode, with the processor feeding back a gain control signal to set the analog system gain prior to the digitizer. Also, in this embodiment, the received energy is processed in a very narrow bandwidth, which improves the quality of the threshold estimate. Digital processing also measures the amplitude, based on the output of the digitizer and the gain control setting. This amplitude measurement value is coupled to a digital comparator and compared against a reference number to determine range. As in the first embodiment, the range measurement is validated by combining with a valid field detection data bit.